The present invention relates to a pneumatic tool.
It is known in the prior art to provide a pneumatic tool having a fluid motor which includes a rotor and passages for connecting the rotor with a source of fluid. A normally open valve member is interposed in the passages and is movable to a closed position in which it interrupts the flow of the fluid to the rotor.
In U.S. Pat. No. 3,749,530, there is described a cup spring which is mounted on the rotor for rotation therewith along the line of the passage. The cup spring is provided with a number of weights. Thus, should the speed of revolutions of the rotor exceed a predetermined level, the weights bend the cup spring so that the latter closes the passage to thereby interrupt the flow of the fluid to the rotor. As a result of the fluid-flow interruption the fluid motor is shut down. In order to renew the operation an operator has to open a corresponding closure of the motor and bend the cup spring back into its normal uncompressed position so as to open the passage for the fluid flow to the rotor. Simultaneously, the operator may find out and eliminate the cause of the undesired increase of the speed of the rotor.
This device is very simple in manufacturing and assembling. It has been found, however, that the cup spring during its bending, in order to close the fluid-flow passage and subsequently shut down the fluid motor, unavoidably abrades small particles which are then aspirated by the motor and can cause damage.
Usually, a fluid motor is very sensitive to any foreign particles, no matter how small they are. Thus, for example, a foreign body in a circumferential gap between a vane and an inner surface of the stator of the fluid motor may cause damage to the motor (i.e. an undesired groove may develop on the highly precise inner surface of the stator).
In U.S. Pat. No. 3,930,764 it has been suggested to utilize a light plate of synthetic plastic material, which is interposed in the fluid-flow passage. The plate is rotatable with the rotor of the fluid motor and prevented against an axial movement relative to the rotor. A plate is provided with a substantially straight wire spring which is connected to a small counterweight. When the speed of revolution of the rotor exceeds a predetermined level the counterweight lifts the wire spring from a corresponding groove on the rotor and the fluid flow under pressure entrains the plate until the latter abuts a partition on the rotor so as to close an inlet through which the fluid flow enters the motor. In such a position the plate stays and prevents any fluid flow to the rotor of the fluid motor.
While in the case of the U.S. Pat. No. 3,749,530 the fluid motor is shut down due to the biasing force of the bend cup spring, the plate of the U.S. Pat. No. 3,930,764 stays in the closed position thereof due to the pressure differential in the passage upstream and downstream of the plate. The plate has a relatively large diameter. On the other hand, the thickness of the plate is rather small, especially if considered in relation to the diameter thereof. Under a relatively high pressure, applied to the plate when the latter is in the closed position, the plate may become tilted or bent so as to cause an undesired leak of the fluid in the fluid-flow passage. Therefore, in order to eliminate this potential danger one has to reduce the pressure on the plate when the latter is in the closed position, which fact considerably reduces the reliability of the closing of the fluid-flow passage by such a plate.